Ball tonometry: A rapid, nondestructive method for measuring cell turgor pressure in thin-walled plant cells

In this article we describe a new method for the determination of turgor pr essures in living plant cells. Based on the treatment of growing plant cell s as thin-walled pressure vessels, we find that pressures can be accurately determined by observing and measuring the area of the contact patch formed when a spherical glass probe is lowered onto the cell surface with a known force. Within the limits we have described, we can show that the load (det ermined by precalibration of the device) divided by the projected area of t he contact patch (determined by video microscopy) provides a direct, rapid, and accurate measure of the internal turgor pressure of the cell. We demon strate, by parallel measurements with the pressure probe, that our method y ields pressure data that are consistent with those from the pressure probe. Also, by incubating target tissues in stepped concentrations of mannitol t o incrementally reduce the turgor pressure we show that the pressures measu red by tonometry accurately reflect the predicted changes from the osmotic potential of the bathing medium. The advantages of this new method over the pressure probe are considerable, however, in that we can move rapidly from cell to cell, taking measurements every 20 s. In addition, the nondestruct ive nature of the method means that we can return to the same cell repeated ly for periodic pressure measurements. The limitations of the method lie in the fact that it is suitable only for superficial cells that: are directly accessible to the probe and to cells that are relatively thin walled and n ot heavily decorated with surface features. It is also not suitable for mea suring pressures in flaccid cells.